Mr Pae's sentence came amid high tension between the US and North Korea, after Pyongyang's third nuclear test.

Mr Pae, believed to be a tour operator of Korean descent, was tried on 30 April. It is not known if he was immediately taken to jail after the sentencing.

'Immediate release'

On Wednesday, US state department spokesman Patrick Ventrell said the country had "long standing concerns" about the lack of transparency and due process in North Korea's legal system and in Mr Pae's case.

Analysis

By Lucy WilliamsonBBC News, Seoul

On the face of it, North Korea's decision to sentence a US citizen to 15 years' hard labour seems to be a direct challenge to Washington: another twist in the cycle of actions and rhetoric that have helped keep relations so tense over the past two months.

But Mr Pae is not the first American citizen to be arrested or tried in North Korea. Over the past few years, Pyongyang has detained two American journalists, a businessman, an English teacher and an activist.

Some were tried and sentenced to hard labour like Mr Pae. But all were released following negotiations - some of which involved unofficial visits by high-profile Americans like former presidents Jimmy Carter and Bill Clinton.

So, while it may seem like another irritant to relations with Washington, the announcement of Mr Pae's conviction might actually be an attempt to draw US negotiators - even unofficial ones - to Pyongyang.

That would give North Korea a domestic propaganda victory, and it might also pave the way for more broader, more official, talks on the wider issues.

At the moment, North Korea is being offered talks on American terms - which include a commitment to dismantle its nuclear weapons programme. This is one way the regime can get a high-profile visitor to Pyongyang without any conditions at all.

"Now that [Mr Pae] has gone through a legal process we urge the DPRK to grant him amnesty and immediate release," Mr Ventrell said.

Sweden, which represents the US in North Korea in the absence of diplomatic ties, had helped to provide counsel to Mr Pae, reports said.

North Korean media reported last week that he had admitted charges of crimes against North Korea, including attempting to overthrow the government.

The 44-year-old was arrested in November as he entered the north-eastern port city of Rason, a special economic zone near North Korea's border with China.

South Korea activists believe he was arrested for taking pictures of starving children.

The Associated Press news agency also reports that he is described by friends as a devout Christian.

'Biggest mistake'

The sentence comes after several months of heightened tensions between the US and North Korea.

The UN expanded sanctions against the communist state in March, in the wake of its 12 February nuclear test and December long-range rocket launch.

Pyongyang reacted angrily both to the measures and the annual US-South Korea military exercises, threatening to attack US military bases and cutting key hotlines with its southern neighbour.

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